Tomorrow is the 200th birthday of a man who changed the world. Charles Darwin provided a biological explanation for how all species evolved on Earth. His 1859 book, "On the Origin of Species," was as revolutionary, and to some as controversial, as any book ever written.

NPR's science correspondent Joe Palca traveled to England and has this report on the evolution of the theory of evolution.

JOE PALCA: To tell the story of evolution, let's start at the beginning - the beginning of my trip to England, that is.

(Soundbite of people talking)

PALCA: It's Friday night, January 16th, and I'm on the campus of the University of Cambridge. We're in Lady Mitchell Hall and Sean Carroll's about to give a lecture. This place is packed.

Sean Carroll's day job is molecular biologist at the University of Wisconsin. But he's got the Darwin bug - he's written three popular books about Darwin and evolution. Is "Darwin groupie" too strong? Well, let's just say he didn't hesitate when he was asked to come to England and kick off the Darwin College Lecture Series.

PALCA: For an hour, Sean told a rapt audience how modern biologists studying evolution at the molecular level are proving Darwin more right than he could possibly have known in his own lifetime. The lecture was the start of a long weekend I spent with Sean, who agreed to help me understand how Darwin's theory evolved.

But before we got to work, Sean was eager for me to meet someone at the reception after his talk.

Mr. CARROLL: 'Cause I'm keen on that and I have notes as to exactly, pages and things.

PALCA: We're in a small room on the third floor of the Cambridge University library. Allison Pearn is with us. She's assistant director of the Darwin Correspondence Project, an effort to sort and publish all of Darwin's letters. Cambridge has a huge collection of them, as well as his field notes and papers.

Sean's ordered up a slew of documents. He wants to show me just how early on Darwin begins to realize that, over time, species change. He pulls out notes that the 27-year-old Darwin wrote on the way back to England on the Beagle. Darwin is speculating on why the different islands of the Galapagos each has a different kind of mockingbird on it.

Mr. CARROLL: Here's the killer: if there is the slightest foundation for these remarks, the zoology of archipelagos will be well worth examining, for such facts would undermine the stability of species. And this is his first observations related to understanding, starting to recognize, that species change.

PALCA: I should point out here that Sean is actually reading from transcripts he's brought with him. Darwin may have been a great scientist, but it takes practice to read his awkward scrawl.

Next, Sean wants me to see a famous page from one of the little red notebooks Darwin filled after he was back in England. It's the famous Notebook B, where Darwin makes a sketch of a tree - the tree of life - with branches from a central trunk, illustrating the radical idea that species evolved from a common ancestor.

But Sean's disappointed. Notebook B is too famous for mere mortals to see. All Allison can show us is a facsimile, but she does hand him the original Notebook N.

Mr. CARROLL: Page 36 - can I touch it?

Ms. PEARN: Yeah, yeah.

(Soundbite of laughter)

Mr. CARROLL: So I can at least tell my kids.

There's the difference, Joe, between facsimile, okay? These little red notebooks with their letters on them. Colored - you can just see, easy to fit in the pocket. Anything occurs to him, flip it open, write on writing, stream of consciousness. And he's burning through these notebooks in 1837 and 1838. It's just the notes are just piling up. His mind is just gushing.

PALCA: Sean says Darwin is now moving to the idea that the forces of nature alone can shape species, and that biology is ruled by natural laws, just as physics and astronomy are. Holding the precious notebook, Sean reads from a transcript.

Mr. CARROLL: We can allow satellites, planets, suns, universes, nay, whole systems of universes to be governed by laws. But the smallest insect we wish to be created at once by special act.

PALCA: Sean says by putting an end to the notion that only a special act of a divine creator could be responsible for the marvelous variety of life on Earth, Darwin has come up with something that will be provocative, upsetting, heretical.

Mr. CARROLL: He knows that everyone wants every being to be specially created. This is what he's up against - the prevailing doctrine of special creation, that species are put instantaneously on the Earth in a particular place for a particular period of time and not related, as he has deduced, by descent from ancestors.

PALCA: Sean says Darwin's ideas continued to evolve, and by 1844 he had written draft language that almost perfectly parallels what will be in "On the Origin of Species" 15 years later. But he was in no rush to publish because he knew he would take a lot of heat once his ideas were loosed upon the world.

Mr. CARROLL: He had the, you could call it the luxury to let this, to have this gestate for 20 years. But, of course, every month he waited there was a risk that someone else would come up with this idea.

PALCA: And of course someone did come up with the idea. Another one of Sean's heroes - a British naturalist named Alfred Russel Wallace, who had traveled to the Malay Archipelago in the 1850s.

Sean was supposed to fly back to the States on Tuesday morning, but when he learns I'm going to see some of Wallace's papers at the Natural History Museum in London, he changes his flight to Tuesday afternoon so he can join me.

At the museum we find more historical treasures. Museum Curator George Beccaloni pulls out another small 19th-century notebook.

Mr. GEORGE BECCALONI (Curator, Natural History Museum): And you might interested to see this; this is Wallace's notebooks from the Malay Archipelago from 1858.

Mr. CARROLL: Oh my God, I didn't know it existed.

PALCA: Sean says Wallace's notes and papers show he was thinking along identical lines to Darwin, albeit 20 years after Darwin's ideas began to coalesce.

Mr. CARROLL: He was finding different species on different islands. They were clearly related because they were all bird-winged butterflies. And what he was thinking was, well, gee, I'm finding these closely allied species near each other, so new species are evolving in the proximity of preexisting relatives. And it's really a parallel line of thought to what Darwin was thinking about in terms of finches and mockingbirds.

PALCA: History has given most of the credit for the theory of evolution by natural selection to Darwin. But Sean says Wallace deserves better. So be forewarned, we have 14 years to get ready for the 200th anniversary of Wallace's birth.

Joe Palca, NPR News.

WERTHEIMER: You can see a slideshow of drawings from the sketchbooks of Darwin and Wallace at NPR.org.

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